Not applicable.
The present invention relates to an improved laser transmitter and, more particularly, to a laser transmitter and method of laser transmitter compensation in which thermally induced errors in the grade of the projected beam of laser light are reduced by monitoring the transmitter temperature and correcting the transmitter operation accordingly.
Laser transmitters are commonly used in surveying and in the construction industry for measuring or checking elevations, grade, dimensions from off-set lines, and the like. It is well known, for example, to use a laser beam transmitter in place of the level instrument. At the location where elevation is to be measured or checked, a target or laser beam detector is employed to intercept the laser beam from the transmitter. The laser transmitter may include rotating optical components which produce a beam that sweeps in a generally horizontal plane. Some such transmitters incorporate visually readable level vials and manually adjustable screw mechanisms to permit the transmitter to be oriented so that the plane defined by the beam is level or is tilted in a desired direction at a desired grade. Other similar transmitters employ a generally conical reflector which intercepts the laser beam and reflects it simultaneously generally perpendicular to the beam in all directions. This stationary reflector has the advantage of simplicity. It will be appreciated, however, that the intensity of the light in the plane will be substantially reduced when it is reflected out of the beam. Still other laser transmitters project a stationary beam that is used as a reference in operations such as for example pipe laying.
While known laser transmitter systems provide many improvements over conventional level and rod survey equipment, they also present certain disadvantages and limitations. For example, the degree of accuracy in establishing a desired beam orientation is dependent on the operator""s skill and judgement in reading the level vials as he operates the adjusting screws. Moreover, where the operator moves away from the device to tend the target or a beam detector, the laser beam transmitter can move out of adjustment, as from being jarred, without the operator""s knowledge so that subsequent measurements are erroneous.
A laser transmitter having significant advantages over earlier prior art devices is shown in U.S. Pat. No. 4,062,634, issued Dec. 13, 1977, to Rando et al, which is commonly assigned with the present application. The system disclosed in the Rando et al patent is one in which orientation of the laser beam reference plane is accomplished automatically. A support frame for the laser source is pivotally mounted on the base frame of the Rando et al device. The support frame carries electrical sensor vials which sense the orientation of the support frame and provide electrical signals used by a feedback control system. The feedback control system activates electric motors to move the support frame into a position in which the vials are leveled. The vials are mounted on the support frame in such a manner that their positions may be adjusted by separate grade motors. When the reference laser plane is to be oriented at an angle to the horizontal, at least one grade motor is actuated by the operator to tilt a vial with respect to the support frame. The feedback control system then reorients the support frame to bring the vial back into its level position, tilting the frame by the desired amount. Other laser transmitters that incorporate level vials to detect orientation of transmitter components are shown in U.S. Pat. No. 5,852,493, issued Dec. 22, 1998, to Monnin, and in U.S. Pat. No. 6,055,046, issued Apr. 25, 2000, to Cain.
While also providing a significant improvement over the prior art, it has been found that laser transmitters of this type may experience significant errors as a result of changes in ambient temperature. It has been found that one source of these temperature induced errors are the level vials in these systems. A level vial of the type used in such transmitters typically comprises an electrically nonconductive vial casing, usually made of glass, that defines an elongated, arcuate chamber which curves generally downward toward its opposite ends. A quantity of electrically conductive fluid is provided in the chamber. Such a fluid may, for example, have a ketone component. A pair of end electrodes electrically communicate with the upper portions of the chamber adjacent its opposite ends and extend toward the central portion of said chamber. A common electrode extends substantially the entire length of the chamber along its lower surface. The quantity of electrically conductive fluid in the chamber is such that an air bubble is left in the chamber, rising to whatever portion of the chamber is uppermost. It will be appreciated that, as the vial is tilted in one direction, the electrical impedance of a path from one end electrode through the electrically conductive fluid to the common electrode will increase, while the electrical impedance of a path from the other end electrode to the common electrode will decrease. When the vial is tilted in the opposite direction, the end-electrode-to-common-electrode impedances change in the opposite fashion. When the two end-electrode-to-common-electrode impedances are equal, the vial can be said to be oriented horizontally. It will be appreciated, however, that other impedance ratios might be defined as horizontal, if desired.
In any event, changes in the ambient temperature of a vial may cause the vial casing to change dimensions and shape. Of particular concern is any asymmetric change in the shape of the chamber, in that this may result in a change in the position of the air bubble and a change in the impedance ratio without any actual change in vial orientation. Vials have, in the past, been thermally insulated. While this reduces short term temperature fluctuations and temperature gradients along the length of the vial, it does not reduce errors stemming from asymmetric changes in chamber shape.
While the level vials account for a significant portion of the temperature dependent transmitter errors, other transmitter components are also a factor, as well. Most mechanical components will expand with increases in temperature. Some components will also change shape with increases or decreases in temperature. Further complicating matters is the fact that components are made of various materials and the materials may have varying coefficients of thermal expansion. These varying coefficients of thermal expansion may cause components that fit together perfectly at one temperature to bind or otherwise fit improperly at other temperatures. Since the various thermally induced errors can be cumulative so that the resulting errors are compounded, and since such errors can vary significantly from one transmitter to the next, a need exists for a laser transmitter and method of laser transmitter calibration in which such errors are eliminated, or at least minimized.
This need is met by a transmitter for projecting a beam of laser light and a method according to the present invention in which thermally induced errors are compensated. The transmitter includes a source of a beam of laser light, a projection arrangement for directing the beam of laser light at a selected grade, a temperature sensor for detecting the temperature of said transmitter, and a temperature correction circuit. The temperature correction circuit includes a look-up table. The temperature correction circuit is responsive to the temperature sensor and adjusts the projection arrangement in dependence upon offset grade values that are stored in the look-up table for a plurality of transmitter temperature ranges. By this arrangement temperature induced errors in the direction of the beam of laser light are compensated.
The projection arrangement includes a level vial. The level vial comprises an electrically nonconductive vial casing defining an elongated chamber which curves generally downward toward opposite ends thereof. A quantity of electrically conductive fluid is provided in the chamber. A pair of end electrodes electrically communicate with the upper portions of the chamber adjacent opposite ends and extend toward the central portion of the chamber. A common electrode electrically communicates with the lower portion of the chamber. The vial is presumed to be level when resistances measured at each end are equal. The magnitudes of these resistances change with temperature, thus providing an indication of temperature. The temperature sensor includes a current sensor circuit for sensing the resistivity of the electrically conductive fluid. The current sensor includes a test resistance connected to one of the end electrodes, and a test circuit for determining the voltage across the test resistance in response to the application of a test signal of predetermined voltage and short duration across the end electrodes of the level vial.
The projection arrangement for directing the laser light at a selected grade comprises a level vial having a quantity of electrically conductive fluid. The temperature sensor includes a circuit for sensing the bulk resistivity of the quantity of electrically conductive fluid. The projection arrangement for directing the laser light at a selected grade includes an arrangement for changing the direction of the beam until the selected grade is reached. The temperature correction circuit includes a circuit for providing the offset grade value thereto. The circuit for providing an offset grade value to the arrangement for changing the direction of the beam includes a look-up table having offset grade values associated with specific temperatures. The offset grade values are specific to the individual transmitter. By this technique temperature induced errors associated with any portion of the transmitter may be compensated. The look-up table has offset grade values associated with at least three specific temperatures. The temperature sensor may alternatively comprise a thermistor.
The source of a beam of laser light may be a source of a rotating beam of laser light defining a reference plane. The projection arrangement directs the laser light at selected grades in first and second orthogonal axes, and the temperature correction circuit includes first and second look-up tables for adjusting said projection arrangement such that temperature induced errors in the direction of the beam of laser light along the first and second axes are compensated. The projection arrangement for directing the beam of laser light at a selected grade may include a generally conical reflector which reflects the beam radially outward, substantially in a reference plane. The projection arrangement directs the laser light at selected grades in first and second orthogonal axes.
The projection arrangement for directing the beam of laser light at a selected grade includes a generally conical reflector which reflects the beam radially outward, substantially in a reference plane. The projection arrangement directs the laser light at selected grades in first and second orthogonal axes. The temperature correction circuit includes first and second look-up tables for adjusting the projection arrangement such that temperature induced errors in the direction of the beam of laser light along the first and second axes are compensated.
A method of calibrating a transmitter for projecting a beam of laser light, the transmitter having a source of laser light, a projection arrangement for directing the laser light at a selected grade, a temperature sensor for detecting the temperature of the transmitter, and a temperature correction circuit, responsive to the temperature sensor, for adjusting the projection arrangement such that temperature induced errors in the direction of the beam of laser light are compensated, and the beam of laser light is thereby directed substantially at the selected grade, comprises the steps of: a.) selecting a plurality of temperatures for which correction will be made; b.) subjecting the transmitter to an ambient temperature equal to the first temperature for a period sufficient to achieve thermal equilibrium; c.) setting the transmitter for projecting the beam of laser light at a specified grade; d.) measuring the actual grade of the beam of laser light; e.) determining the error in the grade of the beam of laser light achieved; f.) determining the grade offset needed to correct for the error in the grade of the beam of laser light; g.) measuring the temperature of the vial at that ambient temperature; h.) storing the grade offset for the first temperature range in a look-up table; and i.) repeating steps a.) through g.) for each of the others of the plurality of temperatures.
The step of selecting a plurality of temperature ranges for which correction will be made may include the step of selecting three temperatures, or it may include the step of selecting five temperatures. The step of setting the transmitter for projecting the beam of laser light at a specified grade may comprise the step of setting the projecting the beam of laser light at a specified grade in a first direction and setting the projecting the beam of laser light at a specified grade in a second direction, orthogonal to the first direction. The step of measuring the actual grade of the beam of laser light may include the step of measuring the actual grade of the beam of laser light in the first direction and in the second direction. The step of determining the error in the grade of the beam of laser light achieved may include the step of determining the error in the grade of the beam of laser light achieved in the first direction and in the second direction. The step of determining the grade offset needed to correct for the error in the grade of the beam of laser light may include the step of determining the grade offset needed to correct for the error in the grade of the beam of laser light in the first direction and in the second direction. Finally, the step of storing the grade offset for the first temperature in a look-up table may include the step of storing the grade offsets for the first and second directions for the first temperature in a pair of look-up tables.
A method of calibrating a transmitter for projecting a beam of laser light, the transmitter having a source of laser light, a projection arrangement for directing the laser light at a selected grade, a temperature sensor for detecting the temperature of the transmitter, and a temperature correction circuit, responsive to the temperature sensor, for adjusting the projection arrangement such that temperature induced errors in the direction of the beam of laser light are compensated, and the beam of laser light is thereby directed substantially at the selected grade, comprising the steps of: a.) selecting a plurality of temperature ranges for which correction will be made; b.) subjecting the transmitter to a first ambient temperature for a period sufficient to achieve thermal equilibrium; c.) setting the transmitter for projecting the beam of laser light at a specified grade; d.) measuring the actual grade of the beam of laser light; e.) determining the error in the grade of the beam of laser light; f.) determining the grade offset needed to correct for the error in the grade of the beam of laser light; g.) repeating steps a.) through f.) for each of a plurality of temperature ranges; h.) constructing a grade offset curve; i.) determining from the grade offset curve the grade offset needed for the temperature at the midpoint of each of the plurality of temperature ranges; and g.) storing the grade offset needed for the temperature at the midpoint of each of the plurality of temperature ranges.
The step of selecting a plurality of temperature ranges for which correction will be made may include the step of selecting three temperature ranges, or the step of selecting five temperature ranges. The step of setting the transmitter for projecting the beam of laser light at a specified grade may comprise the step of setting the projecting the beam of laser light at a specified grade in a first direction and setting the projecting the beam of laser light at a specified grade in a second direction, orthogonal to the first direction. The step of measuring the actual grade of the beam of laser light may include the step of measuring the actual grade of the beam of laser light in the first direction and in the second direction. The step of determining the error in the grade of the beam of laser light may include the step of determining the error in the grade of the beam of laser light in the first direction and in the second direction. The step of determining the grade offset needed to correct for the error in the grade of the beam of laser light may include the step of determining the grade offset needed to correct for the error in the grade of the beam of laser light in the first direction and in the second direction. The step of constructing a grade offset curve may include the step of constructing a grade offset curve for the first direction and constructing a grade offset curve for the second direction. The step of determining from the grade offset curves the grade offsets needed for the temperature at the midpoint of each of the plurality of temperature ranges may include the step of determining from the grade offset curve the grade offset needed for the temperature at the midpoint of each of the plurality of temperature ranges for the first direction and for the second direction. The step of storing the grade offset needed for the temperature at the midpoint of each of the plurality of temperature ranges may include the step of storing the grade offset needed for the temperature at the midpoint of each of the plurality of temperature ranges for the first direction and for the second direction.
Accordingly, it is an object of the present invention to provide a laser beam transmitter which corrects effectively for thermally induced grade errors; to provide such a transmitter in which the temperature of the transmitter is monitored and correction is effected through grade offset values that are stored in a look-up table and that are unique to the errors of the specific transmitter; and to provide a method of calibrating such a laser beam transmitter in which grade offset values are determined specifically for the transmitter of interest and stored in one or more look-up tables. Other objects will be apparent from reference to the accompanying description and claims.